"We do not know who is the other side. They don't even have a final list," he told a packed news conference.

US Secretary of State John Kerry urged both sides "to make the most of this moment" in a statement from Washington.

Kerry urged the opposition to drop their preconditions, but also had hard words for their foes, accusing Assad's forces of deliberately starving besieged towns.

He described the talks in Geneva as being at a "pivotal phase" and said the HNC represented an "inclusive opposition".

De Mistura's office said Sunday that further separate talks would be held on Monday with the opposition and government delegations.

HNC spokesman Munzer Makhous told AFP that the group would stay in Geneva for at least three or four more days.

- Five years -

The Syrian conflict, which began in 2011, has sucked in a wide range of actors from Turkey, Iran and the Gulf states to Western nations and since September, Russia.

The chaos has allowed IS militants to overrun swathes of the country and also in neighbouring Iraq, giving it a launchpad for a series of deadly attacks across the globe including the Paris massacres on November 13 which left 130 dead.

Half of Syria's population have fled their homes, forcing millions to seek refuge in neighbouring countries and also in Europe, where the influx has sparked a major political and social crisis.

The Syrian peace talks, meant to last six months, are part of an ambitious roadmap agreed by all the external powers embroiled in the conflict in Vienna in November, a day after the Paris attacks.

This "Vienna Process" envisages negotiations followed by a transitional government, a new constitution and elections within 18 months, but leaves the question of Assad's future unresolved.

Another thorny issue is which rebel groups will be involved in the talks, although all sides agree on the exclusion of extremists from IS and Al-Nusra Front, Al-Qaeda's Syrian branch.

The powerful Army of Islam rebel group is part of the HNC's delegation to Geneva, and the HNC's chief negotiator and Army of Islam member Mohammed Alloush told AFP Sunday that he would be in Geneva the next day.

The choice of Alloush has been controversial, with Syria's ally Moscow saying it considers the Army of Islam to be "terrorists" and other opposition groups insisting it was unacceptable for the delegation to be led by a member of an armed group.

In a potentially positive sign for the talks, sources said late Sunday that HNC head Riad Hijab, a former Syrian prime minister, was expected in Geneva on Monday.